1. Field of the Invention
This invention relates to photopolymerization reactions and photoinitiator compositions useful therein and is more particularly concerned with the photopolymerization of ethylenically unsaturated compounds using a combination of an aromatic ketone photosensitizer and a substituted urea or amide and with certain compositions comprising mixtures of said compounds.
2. Description of the Prior Art
The use of aromatic ketones such as benzophenone, acetophenone, dialkoxyacetophenones and the like as photosensitizers in the photopolymerization of ethylenically unsaturated compounds is well-known in the art. It is also known that the use of combinations of such aromatic ketones with other photosensitizers will permit photopolymerization processes to occur at a faster rate than is the case if the individual photosensitizers are used alone. Indeed, some photosensitizers, e.g. benzophenone, will not function when used alone. A combination of photosensitizers which is frequently used is a mixture of at least two aromatic ketones, one of which is benzophenone or acetophenone or the like and the other is Michler's ketone [p,p'-di(dimethylamino)benzophenone]. While this type of combination gives excellent results in terms of rates of photopolymerization, it has a number of disadvantages, the chief of which is that the use of the Michler's ketone frequently gives rise to discoloration in the finished polymer.
We have now found certain combinations of aromatic ketone photosensitizers with tetra-substituted ureas and or N,N-disubstituted amides which give excellent rates of photopolymerization when employed in systems involving the photopolymerization of ethylenic double bonds and which are free from the drawbacks encountered with Michler's ketone and which possess additional advantages which will be discussed in detail below.
The use of amides and urea and urea derivatives in polymerization reactions which are activated by radiation of various kinds has been described in the art. U.S. Pat. No. 3,993,549 describes the addition of finely powdered urea to liquid polyene-polythiol systems which are to be cured by ultraviolet or other types of radiation in the presence of photosensitizers such as aromatic ketones.
U.S. Pat. No. 4,088,554 describes the use of amides of .alpha.,.beta.-dioxocarboxylic acids as photoinitiators for the curing of ethylenically unsaturated resins by exposure to ultraviolet light. U.S. Pat. No. 4,153,525 describes the use of a broad class of amides, including N,N-disubstituted amides, for use in the curing of unsaturated polyester resins by exposure to ionizing radiation. U.S. Pat. No. 4,222,835 relates to systems curable by ultraviolet radiation which comprise a polymerizable monomer, thermal initiator, a photoinitiator (which can be an aromatic ketone photosensitizer) and an accelerator which serves to lower the temperature of activation of the thermal initiator. The list of the latter accelerators named in the reference is inclusive of dimethylformamide.
Belgian Pat. No. 884,395 shows the use as comonomers in photopolymerizable systems of tetrasubstituted ureas which contain at least one vinyl substituent attached to a nitrogen atom as well as N-vinyl substituted amides.
So far as I am aware it has not previously been suggested that tetrasubstituted ureas or N,N-disubstituted amides could be used in combination with aromatic ketone sensitizers as photoinitiators or photosensitizers in systems involving photopolymerization of ethylenically unsaturated compounds.